The invention concerns retroreflecting metrology devices used, in particular, to measure deformation of natural structures (the ground, rock formations) or manmade structures (civil engineering works, embankments, cuttings).
Metrology measurements on such structures are conventionally effected by rangefinders which use xe2x80x9cInvarxe2x80x9d (registered trademark) wires. In outline, they entail implanting supports in the structure whose dimensional evolution is to be checked, and wire attachment members are removably mounted on the supports. An adjustment member is used to stretch wires whose ends have been attached to two respective attachment members, measuring (allowing for the length of the wires) the distances between reference points of the attachment members, removing the equipment with the exception of the supports, and repeating the above operations each time the dimensional evolution of the structure is to be checked.
The supports are therefore permanently anchored in the structure but the rest of the equipment is removable. The attachment members have a structure such that merely knowing the distance between the respective attachment points of two members, between which one or more wires are stretched, enables the distance between the reference points thereof to be deduced so that, in the absence of deformation, the measurements are reproducible, even at intervals of several years, and so that the changes in the measured distances are a perfect reflection of the deformations of the structure. To enable measurements to be effected over a wide range of distances, wires of calibrated length can be joined together end-to-end. To obtain great accuracy, an attachment member, one or more wires, and an adjustment member comprising a section of measurable adjustable length and a calibrated tensioner are inserted between two reference points. Thus, the wires are always tensioned with the same tension. As the are made of Invar, their length varies little with temperature. The distance between the two, reference points in a straight line from one reference point to the other is therefore equal to the sum of the lengths of the wires tensioned to the appropriate tension plus the length of the adjustment member plus the length of the attachment member between the calibrated wire and the corresponding reference point. The reproducibility of the measurements is naturally related to the necessity for the reference point to always be identically positioned relative to the supports of all the attachment members.
FIG. 1 illustrates the use of such rangefinders to measure changes in the cross section of the vault of a tunnel, for example a railway tunnel in an underground station.
Seven supports, each of which can removably carry an attachment member, are anchored in the plane of the cross section. A support A is anchored into the keystone, two supports B and C are symmetrically anchored at the corners of the platforms overlying the tracks, two supports D and E are symmetrically anchored to the skewbacks of the vault at the same level, and two supports F and G are symmetrically anchored in the vault or at the springers, at the same level, respectively between the supports D and E and the keystone support A.
The wires are respectively tensioned between B and D, D and F, F and A, A and G, G and E, E and C, C and B, and there are therefore only seven distances to be measured. With seven supports it is theoretically possible to check 21 distances, but in most cases it is sufficient to check 17 distances to obtain a good image of the evolution of the structure.
FIG. 2 shows diagrammatically a device including a support 1 and an attachment member 2 (which are known in the art), and an assembly member 3 by means of which the attachment member is fixed to the support.
The support 1 includes, along a central axis, a support base 11 adapted to be fixed into the structure whose evolution is to be checked, and a receptacle head 12 connected by a transition region 13. The head 12 is adapted to carry the assembly member, externally of the structure. The support base can have projections at its perimeter to anchor it more reliably into the structure, for example a longitudinal succession of annular projections or a screwthread of appropriate profile if it is cylindrical (not shown). It can be anchored by embedding it in resin or in expanding cement, for example. The head 12 is generally cylindrical and has an external screwthread 14 to enable the assembly member 3, which has an inside screwthread 31 for this purpose, to be screwed onto it. The transition region 13 is frustoconical. The head 12 includes a generally cylindrical housing 15 open to the outside at the free end opposite the support base 11 via a frustoconical seat 16 consisting of a bevel at an angle in the range of approximately 40xc2x0 to approximately 50xc2x0. For technological reasons, the outside screwthread 14 preferably does not extend as far as the free end of the support, and in the region of the free end of the support the outside diameter of the support is not greater than the diameter at the root of the screwthread.
The attachment member 2 is made up of several components, and to be more precise includes a bush 21 having a generally cylindrical body adapted to be inserted into the housing 15 and a flange which bears on the frustoconical seat 16. The bush 21 includes a central passage in which is accommodated a fixing rod 22 for an attachment finger 23 fixed to the rod externally of the head 12 to pivot about a pivot 24 perpendicular to the longitudinal central axis of the support 1. The fixing rod 22 has a screwthread along its end region accommodated in the head 12 onto which is screwed a nut for fixing the rod to the bush, bearing against a face of the bush with a washer between them. The pivot 24 is accommodated in a transverse bore in the opposite end region of the fixing rod 22. This region of the fixing rod has an area which bears against the face of the flange of the bush 21 outside the head 12 so that the longitudinal axis of the pivot 24 is at a fixed distance from the outside face of the flange. The point of intersection of the axis of the pivot 24 and of the longitudinal central axis of the support 1 to which it is perpendicular therefore constitutes the reference point of the attachment member 2. The axis of the pivot 24 can therefore be oriented in any direction in the plane parallel to the face of the flange external to the head 12 at a distance equal to that between the reference point and that face, and the attachment finger 23 can be pivoted in a vertical plane through an angle of at least 180xc2x0 if its outside shape is chosen accordingly. The attachment finger 23 has a hook for attaching Invar wires near its end opposite its articulation to the pivot 24.
The assembly member 3 takes the form of a ring forming a nut whose inside screwthread 31 cooperates with the outside screwthread 14 of the support 1. It has an inside surface 32 for clamping the flange against the head 12, with the lateral surface of the flange pressed against the frustoconical seat 16, and the outside surface of the ring can be knurled to facilitate tightening it.
When the respective attachment members of two devices as described above are connected to each other by Invar wires and an adjustment member, and the wires and the adjustment member have been aligned with the attachment finger 23, the distance whose evolution is to be checked is that between the longitudinal axes of the respective two pivots 24 of the two devices.
Repetitive measurements using the above equipment are time-consuming and costly and it is desirable to use simpler and faster means of studying the evolution of the same structures.
However, measurements effected until now by other techniques, in particular optical techniques, supply results that are comparable with those obtained using the equipment just described only after lengthy and difficult (or even impossible) corrections.
The object of the invention is to remedy these drawbacks and to provide optical measuring equipment which gives the same measurement results as the prior art equipment in the absence of evolution in the structures checked, in order to avoid any problem of continuity in studying the evolution of structures previously checked by means of the prior art equipment.
To this end, the invention therefore concerns a retroreflecting metrology target marker device comprising a target member and a support for that target member. The support includes a receptacle head for inserting a target member, and the receptacle head includes a housing into which the target member is inserted from the outside via an outwardly flared frustoconical seat. The target member includes a flange having a spherical annular lateral surface adapted to bear tangentially against the frustoconical seat, and a plate extending out of the support and upstanding perpendicularly on the flange in an equatorial plane of a spherical surface of which the lateral surface of the spherical ring flange is part. The plate has two parallel plane faces, at least one of which is retroreflecting or covered with a retroreflecting coating, and includes a hole passing completely through it and centered on an axis perpendicular to the faces of the plate and passing through the center of the spherical surface.
By virtue of the above structure, it is possible to provide a target member adapted to be inserted not only in a support provided especially for it, but also in a prior art support originally adapted to receive an Invar wire attachment member. IN addition, measurements can be performed using a theodolite, to obtain identical measurement results for the same structure, if it has not undergone any deformation. Thus, the dimensional evolution of the structure can be checked without aberrant results or interruptions due to changing the measuring method.
The device according to the invention can also have one or more additional features. The hole passing through the plate can have at least one frustoconical region through which it opens to the outside, in a flared fashion, to optimize inclined sighting. The hole passing through the plate can have two frustoconical regions through which it opens to the outside on respective opposite sides of the plate, in flared fashion, to optimize inclined sighting. Both faces of the plate can be retroreflecting or covered with a retroreflecting coating. The hole passing through the plate can have at least one frustoconical region through which it opens to the outside, in flared fashion, and the inclination of the wall of the frustoconical region to the axis on which the hole is centered can be from approximately 35xc2x0 to approximately 45xc2x0. The plate can be upstanding on a circular increased thickness portion of one face of the flange. The target member can include a generally cylindrical body joined to the flange and adapted to be inserted into the housing of the receptacle head of the support. The support can include a support base adapted to be anchored into a structure on which metrology measurements are to be effected. The device can also include an assembly member in the form of a ring forming a nut having an inside screwthread cooperating with an outside screwthread of the receptacle head of the support and an inside surface for clamping the flange against the head, and the spherical surface can be part of a sphere having a diameter from approximately 25 mm to approximately 35 mm.